When I was starting out, I always tried to look at things in a real world example -- basically, in layman's terms. My understanding of bank switching was this;
Imagine you have 4 cupboard doors -- you store the majority of dinnerware in the first, the second has glasses, the third has dessert plates/bowls, the fourth has coffee/tea cups.
For the majority of your dinner party, you will access the first cupboard, where the majority of your dinnerware is stored. However, you are in need of glasses and have asked a guest to get them for you -- but your guest doesn't know what cupboard to go to to find glasses, unless you tell them -- if you don't they will come back and say "the glasses are not in cupboard 1" (they looked in cupboard 1 because it was the closest cupboard -- and they're not smart enough to go to the next cupboard -- unless you upgrade your guests to 18F's!). You then tell your guest that the glasses are in cupboard 2 -- they can then find them when they go to look for them.
So, if you don't tell the microcontroller where the glasses are stored, it will open the cupboard it is closest to, and if the glasses are not there, it will not be able to find them.
When you break it down to it's simplest form, it's quite easy to understand.
"Microcontroller -- I want glasses -- they are in cupboard 2 -- go get them."
"Microcontroller -- I want dessert bowls -- they are in cupboard 3 -- go get them."
"Microcontroller -- I want the SFR 'my_var' -- it is in Bank 3 -- go get it."